1. Field of the Invention
The present invention relates to a technology for correcting an attitude of an optical element in an optical scanner.
2. Description of the Related Art
Tandem-type electrophotographic image forming apparatuses (e.g., a copying machine, a printer, a facsimile machine, a plotter, or a multifunction product) include a plurality of image carriers (e.g., photosensitive drums) for different colors. Laser beams are irradiated on surfaces of the respective image carriers to form latent images. In one example, line scanning is performed by laser beams emitted from a plurality of laser units to scan latent images formed on four aligned photosensitive drums, thereby developing the latent images on the photosensitive bodies with development agents of different colors (e.g., toners of yellow, magenta, cyan, and black) and visualizing them, respectively. Thereafter, a transfer material, e.g., a recording sheet, is sequentially conveyed by belts to transport units for the respective image carriers, visual images of the respective colors formed on the image carriers are registered and transferred onto the transfer material, and images transferred onto the transfer material are fixed, thereby obtaining a multicolor image.
Such a tandem image forming apparatus is generally provided with a plurality of optical scanners, which operate independently, corresponding to the respective photosensitive drums, to perform latent image scanning on the photosensitive drums. However, the conventional tandem image forming apparatus has the following disadvantages. An optical scanner using an optical deflector constituted by a polygon mirror and a motor that drives the polygon mirror is relatively expensive. If the optical scanners are provided independently to correspond to the respective photosensitive drums, component costs and manufacturing costs increase. In addition, large space is required to install a plurality of optical scanners each including the optical deflector to correspond to the respective photosensitive drums. As a result, the overall image forming apparatus becomes large. To solve these disadvantages, i.e., to reduce the cost and size, an image forming apparatus configured as follows has been realized. As means for performing an optical scanning on a plurality of photosensitive drums, an optical deflector common to a plurality of laser units is employed. In other words, a single optical deflector simultaneously deflects and scans laser beams from the respective laser units, and irradiates the laser beams on a plurality of photosensitive drums, thereby performing the optical scanning on the respective photosensitive drums.
This conventional image forming apparatus includes optical systems configured as follows. The optical systems, which are arranged symmetrically around the optical deflector in two directions, introduce the laser beams emitted from the laser units and deflected and scanned by the optical deflector onto corresponding scanning target surfaces, and form images, respectively. All components of the optical scanner including these optical systems are stored in a single housing. Therefore, as compared with the conventional apparatus that includes a plurality of optical scanners, the number of components and installation space can be reduced. It is thereby possible to reduce the cost and size of the image forming apparatus.
Meanwhile, such an image forming apparatus normally includes a laser unit for black and three laser units for other colors (e.g., cyan, magenta, and yellow). In a monochrome mode for forming a monochrome image, the image forming apparatus only uses the laser unit for black. In a color mode, the apparatus uses all of the laser units. The apparatus simultaneously deflects the laser beams from these four laser units using a single optical deflector, irradiates the laser beams onto the four photosensitive drums through their corresponding optical systems, and performs the optical scanning on each of the photosensitive drums. To irradiate the laser beams onto the four photosensitive drums and form latent images, the apparatus includes a synchronous sensor that synchronizes write positions in main scanning directions of the laser beams from the four laser units. Using output from this synchronous sensor, timings of scanning the four respective colors are synchronized with one another.
The laser beams are thus transmitted onto scanning target surfaces of the respective photosensitive drums and imaged by the transmission and imaging optical systems provided for each of the laser units. Therefore, scanning lines on the scanning target surfaces formed by the respective laser beams are influenced by characteristics and geometrical arrangements of optical components that constitute the corresponding optical systems, and are therefore different in scanning characteristics. When imaging positions of the laser beams in a sub-scanning direction largely differ, a deviation is caused among the respective colors of a final image, and thereby causes image degradation. Conventionally, the image degradation resulting from color registration deviation is prevented by, for example, detecting the deviations among the respective colors and adjusting write start timings in the sub-scanning direction of the respective laser beams.
However, even if the scanning characteristics that cause the image degradation resulting from the color registration deviation or the like are initially adjusted so as not to cause the color registration deviation, positions of images drawn on the respective photosensitive drums are gradually deviated by a change in attachment positions of the components and a change in shapes of the components themselves due to heat generated by the apparatus or the like, that is, by a change in attitudes of the components. As a result, the image position deviation (hereinafter, displacement) or, in case of the full-color image forming apparatus, the color registration deviation occurs. Furthermore, the scanning lines formed by the respective lasers are influenced by the characteristics and geometric arrangements of the optical components that constitute the corresponding optical systems. Accordingly, the change in the attachment positions of the components caused by heat generation of each scanning line, and the change in the shapes of the components themselves differ among the scanning lines.
With reference to FIGS. 7 and 9, one example of the image displacement due to the change in the attachment positions of the optical components, i.e., the change in the attitudes of the optical components that occurs to the conventional optical scanner is explained.
In FIGS. 7 and 8, reference numeral 203 denotes a single optical deflector arranged in the optical scanner. Reference symbols 204-a and 204-b denote fθ lenses also referred to as first imaging lenses, and reference symbols 205-a and 205-b denote elongated toroidal lenses. In addition, reference numeral 207 denotes an optical housing that stores optical components such as the optical deflector 203, the fθ lenses 204-a and 204-b, and the toroidal lenses 205-a and 205-b, and reference symbol 207A denotes a cover that covers up the optical housing 207.
As shown in FIGS. 7 and 8, the optical deflector 203 is a rotating polygon mirror that includes a plurality of planes of deflection. The optical deflector 203 is driven to rotate at a constant velocity by a polygon motor (not shown). Heat is generated from the polygon motor with passage of an operating time of the optical deflector 203, that is, an increase in the number of rotations of the optical deflector 203. The heat generated from the polygon motor is transmitted not only through the optical deflector 203 directly to the optical housing 207 but also through an air current generated by the optical deflector to the optical housing 207. Accordingly, a temperature distribution is generated in the optical housing 207 according to a distance from the optical deflector 203 and a degree of heat transmission.
The temperature generated in the optical housing 207 deforms the optical housing 207 from a state shown in FIG. 7 in which the optical scanner is not used yet to a state shown in FIG. 8. At this time, attitudes of folding mirrors 206-a and 206-b serving as optical components arranged on both ends of the optical housing 207, respectively are changed. Accordingly, exposure positions of beams L2-a and L2-b reflected by the folding mirrors 206-a and 206-b on respective photosensitive drums 201 and 202 are changed.
This state is explained with reference to FIGS. 9A and 9B. In FIGS. 9A and 9B, the folding mirror 206-a (hereinafter, simply mirror 206-a) arranged on the right side out of the mirrors 206-a and 206-b shown in FIGS. 7 and 8 is typically explained below.
FIG. 9A depicts a state in which optical components within the optical scanner are located at normal positions corresponding to FIG. 7. In FIG. 9A, the laser beam L1 emitted from a laser unit (not shown) is struck against a reflecting surface 206a of the mirror 206-a, reflected by the reflecting surface 206a at an angle θ, and exposed on a surface of the photosensitive drum 201. Upper and lower ends of the reflecting surface 206a of the mirror 206-a are supported by reception surfaces 208-a and 208-b formed integrally with the optical housing 207, respectively. Plate springs 220 serving as urging means or urging members are provided to correspond to both ends of the mirror 206-a formed to extend from a front to a depth side in FIG. 9A, respectively. In a back view, a generally central portion of the mirror 206-a is urged by tip ends of the plate springs 220 in a direction in which the reflecting surface 206a of the mirror 206-a contacts with and is pressed against the reception surfaces 208-a and 208-b. By doing so, the mirror 206-a is held by the reception surfaces 208-a and 208-b so as not to change the attitude and arrangement position of the mirror 206-a by an ordinary vibration or external force transmitted through the optical housing 207 from an outside of the optical housing 207.
For brevity of illustration of the conventional technique as well as embodiments of the present invention to be explained later, hatching of the mirror 206-a and the plate springs 220 are not shown in all drawings except for FIG. 9A.
FIG. 9B depicts a state after some time has passed from when the conventional optical scanner started operating. This corresponds to the state when the optical housing 207 is thermally deformed as shown in FIG. 8 and the attitude of the mirror 206-a is changed (the mirror 206-a rotates in an arrow direction in FIG. 9B). The attitude of the mirror 206-a is thus changed or rotationally displaced from the state indicated by a solid line in FIG. 9A by as much as α as indicated by a solid line in FIG. 9B. The laser beam L1-a from the laser unit is thereby changed to a laser beam L1-a″ folded at a reflection angle that is changed from θ to θ-2α, the laser beam L1-a″ is irradiated and exposed on the photosensitive drum 201. Therefore, the exposure position of the laser beam in, the sub-scanning direction on the photosensitive drum 201 is deviated. Thus, the change in the attitudes of the optical components such as the folding mirrors deviate the intended exposure position.
As shown in FIG. 8, the mirror 206-b arranged on an opposite side to the mirror 206-a across the optical deflector 203 mainly differs from the mirror 206-a only by a rotational displacement in an opposite direction to that of the mirror 206-a. Similarly to the mirror 206-a, the exposure position of the laser beam in the sub-scanning direction on the photosensitive drum 202 is deviated as will be readily understood by a person having ordinary skill in the art. Therefore, the deviation of the exposure position for the mirror 206-b will not be further explained herein. Needless to say, the optical housing 207 is not always deformed ideally, i.e., horizontally uniformly in amount as shown in FIG. 8 because of differences in the shape of the optical housing, the arrangement of the optical components, and the like, as is explained later.
An image forming apparatus that employs this optical scanner is often confronted by a disadvantage of the image displacement or color registration deviation in case of the full-color image forming apparatus due to changes in positions of laser beams while the apparatus is in use. To solve this disadvantage, the following techniques for adjusting the optical components within the optical scanner are known, as disclosed in, for example, Japanese Patent Application Laid-open Nos. 2001-142012, H9-193463, H11-326804, 2004-258182, and Japanese Patent No. 2858735.
According to the technique disclosed in Japanese Patent Application Laid-open No. 2001-142012, scanning time for a scanning performed between two photodiodes is measured, a difference between the measured scanning time and specified time is detected, and mirrors within an optical scanner are rotated by a motor according to a detection result. Feedback control is performed to return the scanning time for the scanning between the two photodiodes to the specified time.
According to the technique disclosed in Japanese Patent Application Laid-open No. H9-193463, a sub-scanning position of a light beam incident on a photosensitive drum is detected to thereby allow a light beam to be incident on a normal sub-scanning write position.
According to the technique disclosed in the Japanese Patent Application Laid-open No. H11-326804, a line drawn on a transfer material is read by a detection sensor, positions of optical components within an optical scanner are changed according to a detection result, and an irradiation position of a light beam on a photosensitive body is thereby adjusted.
According to the technique disclosed in Japanese Patent Application Laid-open No. 2004-258182, a mark drawn on a transfer and transport belt is read by a detection sensor, positions of optical components within an optical scanner are changed according to a detection result, and an irradiation position of a light beam on a photosensitive body is thereby adjusted.
Image forming apparatuses for full-color images have become popular, and there is a demand for further improvement in image quality and acceleration of printing speed. Before realizing high quality image, it is necessary to solve the following disadvantage. If four color images are not accurately registered on the transfer sheet, color registration deviation occurs and a quality of a final image is degraded. To realize both the high quality image and the acceleration of the printing speed, an image forming apparatus that includes an automatic color registration deviation correcting function is normally known as disclosed in Japanese Patent No. 2858735.
The conventional optical scanners and image forming apparatuses including the techniques explained above have the following disadvantages. Attachment positions and shapes of the constituent components and the like are changed due to a heat generated by a unit such as the optical deflector arranged in the optical scanner, transmission and conduction of the heat from a fixing unit or the like. Positions of images drawn on the respective photosensitive drums are gradually displaced. Accordingly, a displacement or color registration deviation in case of a full-color image forming apparatus occurs.
Moreover, to accelerate the printing speed, a driving speed of driving the optical deflector in the optical scanner and a driving speed of driving the transfer unit are accelerated. However, if the optical deflector driving speed is accelerated, in particular, a heat quantity of a motor that drives the optical deflector is increased. The heat generated by the motor adversely influences the other optical components (e.g., a light source, a coupling lens, fθ lenses, elongated lenses such as toroidal lenses) stored in the housing of the optical scanner.
To solve these disadvantages, the techniques disclosed in the above Patent documents are proposed. These techniques are roughly intended to detect light irradiation positions by some means or other, displace the optical components within the optical scanner according to the detection result, and obtain a normal image.
Accordingly, it is necessary to provide at least detection means, arithmetic means for calculating the detection result, an actuator that controls the optical components, and a driving mechanism for the actuator. Pixel densities of currently available image forming apparatuses are mainly around 600 dots per inch (dpi) or 1200 dpi. If the pixel density is around 600 or 1200 dpi, an image is drawn at intervals of 42 or 21 micrometers. It is, therefore, essential to control the drawing interval to be equal to or smaller than 42 or 21 micrometers. To do so, corresponding highly accurate components are required, thereby disadvantageously increasing the cost of the apparatus.
With the technique for drawing the mark on the transfer sheet or transfer body and controlling the optical components according to the marking result, processing time for drawing the mark, erasing the mark, calculating the result etc. is necessary. A user needs to wait during this processing time. Thus, the conventional image forming apparatus is inferior in user friendliness.
Furthermore, with the automatic color registration deviation correcting technique, since the pattern is formed on the belt, detected, and then corrected, the image cannot be printed out during this time. It is, therefore, undesirable to frequently make such an automatic color registration deviation correction that can cause an increase of downtime.